It's possible, but not with the current system in the US (mainly). An overseeing sports association is needed like the FIFA for football (soccer) and the promotions like UFC or Strikeforce have to go. That way there is only the sport of MMA and lots of fighters. Since there's less profit for everybody that way it will probably never happen. Actually the only way I can see it happen is if UFC (or another company) gets big enough to be the only MMA organization in the whole world.

Chuck Liddel from the UFC (wearing a UFC t-shirt to the ring) entered the Pride 2003 Middleweight Grand Prix tournament. He defeated Overeem in the first round and then got clobbered by Rampage (Chuck's corner threw in the towel).

it's possible if organizations agreed, but the UFC would NEVER do such a thing, so even tho it techniqually could happen, it never will, too much for the UFC to lose.

The UFC *already* technically did. Chuck v. Rampage I was in Pride.

Chuck was under contract to the UFC, Rampage was under contract to Pride. Chuck fought in Pride representing the UFC as a UFC fighter (w/ Dana alongside him). 2003. happened. UFC entered their fighter in their major competitors tournament.

You can also add the example of the now defunct Affliction organization and Strikeforce co-promoting.

Arlovski fought on the Kimbo CBS card against an Strikeforce fighter. He was on "loan" from Affliction which was his organization at the time. Babalu (Strikeforce) fought on Affliction cards against Affliction fighters.

Just another example of fighters from different organizations fighting, which was your inital question.

Chuck v. Rampage (after he beat Overeem) was the most notable exampe since it was the UFC vs. Pride, both were major organizations and major competitors, and major stars for each organization (like Superman vs. Spiderman).